In the graphic arts industry, and particularly in lithography, printing plates are usually imaged by exposing them through a film containing a line or half-tone image. At the outset, the original material or "copy" may be in the form of a continuous tone photograph, drawing or type proof. This is then reduced to a black and white transparency of the correct size using a camera or scanner and, during this step, continuous tone images may be converted to half-tone. This transparency, however, is seldom in a form suitable for the exposure of a plate and a contact or first generation copy is made, for example, to combine line and half-tone films in register, to produce a single film from stripped flats, to strengthen half-tone dots or to make spreads or chokes. The first generation copy may then be printed on another film in contact to give a second generation copy, and this even to a third generation before a film suitable for use in plate exposure is obtained.
In the contact printing operation, a target film is placed over a sheet of unexposed film in a vacuum frame. After vacuum has been applied and the films are in good contact, the film is exposed and then processed. Contact printing is usually carried out with silver halide films that can be either negative or positive working. However, photosensitive films, either photohardenable or photosolubilizable, can also be used. They consist typically of a transparent polyethylene terephthalate support, on one side of which is an opaque photosensitive layer covered by a thin oxygen impermeable barrier layer. Actinic light forms relatively soluble and insoluble regions in the photosensitive layer and the soluble regions can then be washed out in a suitable solvent to leave a relief image of the target. It was found in practice that the use of such films often results in an unacceptably high level of pin holes in opaque areas of copies which resulted from residual opaque specks in supposedly clear areas of the preceding generation.
Pinholes in opaque areas result from dirt carried by or pressed into the surface of soluble regions of the preceding generation, for example, during vacuum draw down. Such dirt can retard the penetration of developer in its immediate area and result in a residual speck of underdeveloped photosensitive material which will copy as a pinhole.